The opposite of sincerity is showing off. If someone does something with a bad intention to show off in front of others and impress them, they will not get rewarded. Whenever we do a good deed, we should be sincere about it and do it only for the sake of Allah, not to show off to others or to seek praise or money. So, the next time you do a good deed, stop and think about why you are doing that good deed. Are you sincere about doing it, or are you doing it to impress someone else and maybe show off? We are encouraged to be humble and not show off. Allah loves those who are humble. We should constantly please Allah and not show off.